A spark plug is known, which includes a center electrode, a ground electrode and a needle-like shaped spark section disposed on an inner surface (one side surface) of an end portion of the ground electrode facing the center electrode so as to define a spark gap between the spark section and the center electrode. (See e.g. Patent Document 1.) Herein, the needle-like shaped spark section refers to that, for example, having a protrusion length of 0.6 to 1.6 mm from the inner surface of the ground electrode and an outer diameter (or protruding end face diameter) of 0.5 to 1.2 mm. The spark plug with such a needle-like shaped spark section allows the ground electrode to be located away from the spark gap and reduces the tendency that a flame core generated in the spark gap comes into contact with the ground electrode in the initial stage of flame growth as compared to conventional spark plugs. This makes it possible to decrease a so-called quenching effect of interfering with the flame growth by heat loss upon contact of the flame core with the ground electrode and thereby makes it possible to improve the ignition performance of the spark plug.
The spark section is generally formed using a noble metal having high resistance to spark wear by a concentration of spark discharges. However, there is a large difference between the linear expansion coefficient of the noble metal and the liner expansion coefficient of e.g. nickel-based alloy material commonly used for the ground electrode. If these materials are simply joined together, a crack or separation may occur in the joint between the materials under the influence of thermal load by cooling/heating cycles. Against such a backdrop, Patent Document 1 teaches that the spark section has a noble metal member and an intermediate member having a linear expansion coefficient between those of the noble metal member and the ground electrode and joined to the noble metal member and to the ground electrode so as to increase the joint strength between the spark section and the ground electrode.
In recent years, the combustion conditions of an engine have become increasingly strict for high engine performance and fuel efficiency. This leads to an increase in the amount of heat applied to the ground electrode of the spark plug and an increase in the influence of thermal load applied to the spark section by cooling/heating cycles. As the noble metal member gets heated to a high temperature, it becomes likely that the noble metal member will be consumed by oxidation to cause a deterioration of spark wear resistance. Various modifications have heretofore been made on the spark plug so as to enable rapid heat radiation from the spark section, such as by providing a core material of high thermal conductivity in the ground electrode or by increasing the heat radiation ability of the intermediate member to enhance heat radiation from the noble metal member.
On the other hand, the engine has an exhaust gas passage (exhaust pipe) equipped with a three-way catalyst. This three-way catalyst is activated at a high temperature so as to purify exhaust gas. It is thus common to perform engine control (so-called retard ignition control) that retards the ignition from the normal ignition timing and thereby raises the exhaust gas temperature under low-temperature conditions such as during engine start (e.g. for a lapse of 1 to 2 minutes from turning on the ignition key) so that the high-temperature exhaust gas can be fed to the catalyst to allow early activation of the catalyst and to reduce HC emissions by secondary combustion.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-134209.